it is not the subject issue ... there are 3 parts to consider: one is the subject: either はor が。 2nd is the topic as an object, this is marked with が 3rd is the action (if you focus on it, rather than the subject it is が）. if you ask: "Who is the student" (because you are on a theatrical play and don't know, who took which part.) The answer might be: [大輔が学生です」 Because the topic is Daisuke, who actually plays the role. And the 「が」indicates that he is the real topic. Same setting and you ask; "Who of you is playing the role of the student" The answer might be: [大輔は学生です」 Because you asked for the the student. And the topic of the answer ist that the student is Daisuke. mehhhhh Rather difficult to explain I hope for more answers ...

I guess there have been similar questions in italki before but I recently commented on this matter. I found some useful sites. Follow them from the following link. http://www.italki.com/question/300530

the particle ga marks the subject of a sentence especially when the information is being introduced for the first time. "asoko ni atarashii resutoran ga arimasu ne? " there's a new restaurant over there right? "sono resutoran wa oishii desu" that restaurant is really good